E N E R I N F O A D V I S O R S E R I E S

Selecting Energy-Efficient Windows

Key Points

• Improving the energy performance of windows is important, since windows are the least insulated surface of most homes.

• Energy features such as low-e coatings, argon gas fill and insulated spacers can double the insulation value of a standard double-pane window.

• Energy-efficient windows improve comfort and experience less condensation.

• Savings from energy-efficient windows usually repay any additional purchase costs within a few years.

IntroductionWindows are a long-term investment. Smart shoppers purchase energy-efficient windows

to protect themselves from future energy cost increases and improve comfort for years to

come. Today’s energy-efficient windows offer superior performance at a reasonable price.

Energy-efficiency features available for new or replacement windows include:

• low emissivity (low-e) coatings and films

• inert gas between the panes of thermopane glass units

• insulated edge spacers

• insulated frames

• additional layers of glazing

Energy-Efficient Window Features

High Solar heat gain

Insulatededge spacer

Insulated frame

Inside

Gas-filled airspace

Low-e coating

Durable weather seal

Outside

Why Does Condensation Appear on Windows?Condensation appears on windows because they are

usually a home’s coldest interior surface during the

winter months. If the window glass cools interior air

below its dew point, water droplets from the air begin

to appear on the glass.

Window condensation can seriously damage

window frames, trim, and drywall. In homes with

low-quality windows, increasing ventilation rates

alone may not solve window condensation problems

without reducing humidity below healthy levels. High

ventilation rates also increase heating costs.

The higher insulating value of an energy-efficient

window “warms up” the glass, making condensation

much less likely. Warmer glass is also more comfort-

able to sit near on cold days.

Window condensation after tight-fitting curtains are raised. The window pane on the left has low-e with argon gas fill. The fogged-up window pane on the right is conventional glass.

Avoid mould and moisture damage from condensation by choosing energy-efficient windows.

What is a “Low-e” Window?Low emissivity, or “low-e” coatings, are extremely

thin selective metal films applied to one of the inner

surfaces of thermopane window units (usually the

interior pane in heating climates). Low-e coatings are

engineered to block heat from radiating out of the

home, yet still allow solar energy to enter. Capturing

as much solar energy as possible is especially

important for south, southeast, and southwest

facing glass. Low-e coatings are also available as a

film suspended between glass panes.

Low-e coatings should be specified for all new

or replacement windows. The R-value of a low-e

thermopane is approximately 50 per cent higher than

a standard thermopane’s R-value.

Solar Radiation

Glass

Sun

Low-e Coating

Home Radiation

Low-e Window

Why Use Argon Gas?Replacing the air in a sealed window unit with a

heavy inert gas, usually argon, decreases window

heat loss by reducing air circulation (convection)

between the glass panes. Argon gas increases a

thermopane unit’s R-value by about 30 per cent.

The leakage rate of argon gas fill is much too low to

affect window performance unless the thermopane

unit’s edge seal fails.

Argon gas being added to sealed window units.

Why Use Insulated Spacers?Condensation usually begins at the bottom edge

of the window and then extends up the sides. This

occurs because the metal spacer between the glass

panes of a conventional thermopane window unit

conducts heat very well. As a result, temperatures

at the edge of the glass are much lower than the

temperature in the middle.

Most window manufacturers now offer insulated

spacers. Insulated spacers raise the temperature of

the glass near the frame, saving energy and reducing

edge condensation.

Type R-Value1 Solar Heat Gain

Coefficient2

Double 2.0 .78

Double, low-e3 2.9 .75

Double, low-e, gas filled 3.7 .75

Triple 3.1 .70

Triple, low-e, gas filled 6.0 .50

All windows include insulated spacers 1. R-Value is for centre of glass 2. The higher the number, the greater percentage of solar energy admitted 3. Low-e film emissivity is .20

Window glass units being assembled using insulated spacers.

Is Triple Glazing Still Worthwhile?Double-glazed thermopane window units with low-e,

argon gas, and insulated spacers have a better energy

rating than conventional triple-glazed windows.

Triple-glazed windows are also more expensive, since

a heavier frame and sturdier hardware are required

to accommodate the extra thickness and weight of

the third layer of glass. Very high performance triple-

glazed windows with low-e–coatings and gas fills are

available but are usually not an economical choice

for Nova Scotia’s relatively mild climate.

Are All Frames the Same?No. Frames can affect a window’s energy perfor-

mance. Standard wood and vinyl frames have a

similar insulation value. Insulated fibreglass frames

or foam-filled vinyl frames have higher R-values.

Window frames made of several materials, such as

fibreglass and wood or fibreglass and vinyl, are also

available. Metal and metal-clad frames generally

have the lowest insulating value.

Cut-away view of a vinyl window frame.

Are Some Window Styles More Efficient?Air leakage, particularly between the sash and

frame, affects a window’s energy performance.

Fixed windows are nearly always tighter than similar

windows that open. Specify as many fixed windows

as possible, keeping in mind the need for summer

ventilation and fire safety. Operable windows

usually cost $50 to $100 more than a fixed window

of the same size. Some clever homebuyers offset

the small added cost of high-performance glass with

the money saved by buying fewer operable windows.

Casement and awning windows with compres-

sion seals offer positive closure and seal better

than vertical or horizontal sliders with sliding seals.

They have lower air leakage rates and are more

energy efficient, particularly as the seals of sliding

windows wear over time.

When shopping for windows, examine the

weather-stripping system carefully. Is the weather

stripping flexible? Is it replaceable? Is there more

than one layer of weather stripping between the

sash and the frame?

Should I Replace My Existing Windows?When purchasing a new window, the additional

cost of upgrading to energy-efficient thermopane

units is a sound investment that will be quickly

repaid. However, savings are usually not large

enough to justify replacing a serviceable window

solely to conserve energy. Common reasons for

window replacement include breakage, moisture

damage, rot, seal failure, excessive condensation,

or uncomfortable drafts.

If the basic window frame is sound, many

homeowners choose custom-made vinyl inserts

sized to fit within the existing window frame. Inserts

usually reduce the size of the glass opening by one

to two inches in each direction. Window inserts can

be installed without disturbing the existing interior

and exterior window trim and make a window up-

grade quick, inexpensive, and neat. However, inserts

cannot correct any air leakage between the outside

of the existing window frame and the rough framing.

Additional air sealing must be undertaken to cure

any air leakage around the perimeter of the existing

window frame.

Air Leakage

Quality windows are tested and rated for air leakage, wind resistance and water penetration. When

comparing windows, ask for an air leakage rating of A3, the highest rating.

Fixed Sashless SliderVertical SliderCasementAwning

Very Good Good Poor

Is There an Energy Rating System for Windows?

The international Energy Star® symbol

helps consumers identify products that

are among the most energy efficient on

the market. Choosing an Energy Star®-

labelled product over a conventional model is nearly

always a sound investment and reduces energy

costs, outside noise, and condensation, while also

improving home comfort. Only Energy Star®-

qualified windows are eligible for rebates under

the federal Eco-Efficiency and the Nova Scotia

EnerGuide programs.

There is also an energy rating (ER) system that

assigns a specific number to a window, incorporating

performance parameters such as low-e coatings, gas

filling, air space width, spacer type, number of layers

of glazing, etc. The higher the ER score, the better

the window will perform. ER numbers make it easy

to compare window performance, but unfortunately,

few windows sold in Atlantic Canada have been

rated under this system.

What is the A440 Rating System for Windows?The Canadian Standards Association (CSA) A440

standards allow window manufacturers to test and

rate their windows against specific performance

criteria. The higher the number attached to each

letter, the better the performance of the tested

window will be.• “A” rating for air tightness has three categories,

A1, A2, and A3.• “B” rating for water has categories from B1 to B7.• “C” rating for wind load resistance has categories

from C1 to C5.

This rating system is a useful tool for accessing

important window performance measures. Windows

with the highest ratings available (A3, B7, and C5) are

wise choices for homes in windy, exposed, or coastal

locations with unusually harsh weather conditions.

How Much Can I Save?Computer software developed to calculate energy

ratings can compare the heat loss of a standard

thermopane to a low-e gas-filled window for Halifax’s

climate. Annual savings for a window with a glass

area of 1 square metre range from $9.94 if it faces

south to $12.21 if it faces north. Replacing an older,

less airtight window in an existing home would save

much more.

Window orientation Estimated annual savings per

window with low-e gas fill

North facing $12.21

East facing $10.62

Southeast facing $9.94

South facing $9.38

Southwest facing $9.94

West facing $10.62

Based on electric heat and a 1 square metre window area.

TIP:

For more information about rebates

for Eco-Efficiency and EnerGuide

programs visit www.conservens.ca

or call 1-800-670-4636

How Can I Prevent Air Leakage Around the Window?Comfort and window performance will suffer unless

the gap between the window and rough framing

is sealed with a material that prevents air leakage.

Glass-fibre insulation does not prevent air leakage

unless it is combined with another material that can

stop air movement. Methods builders use to seal

this area include the following:

• Backer rod and caulking: Lengths of round foam

rod (available in different diameters) are tightly

wedged in the rough opening between the outside

of the window and the framing. Any remaining

gaps are caulked.

• Expanding foam: Select a low-expansion foam

with a delivery system, usually some type of gun,

that allows for accurate and neat application of the

foam. Spray cans of foam can be very difficult to

work with. High-expansion foam can damage glass

or distort the window frame enough to keep the

window from opening and closing properly.

• Drywall or plywood return: Drywall or plywood is

installed as close to the window as possible, and

any remaining gap is sealed to the window frame

to create a continuous air barrier.

• Contractors sheathing tape: Air barrier tape is

used to connect the plastic or drywall to the

window frame. The tape is then covered with

window casing. This system depends on the tape

continuing to stick properly over the long term.

• Poly wrap: A 25- to 30-cm (10”–12”) strip of

polyethylene is sealed to the side of the window

frame and then wrapped back and sealed to the

wall’s air barrier. This approach works very well

for window installations in new homes but is

labour intensive.

How Do I Avoid Water Leaks?It is extremely important that windows be properly

installed in both new and retrofit situations. Poorly

installed windows can lead to frustrating and

destructive leaks that can ruin drywall and nearby

wood surfaces. Steps recommended for trouble-free

window installation include the following:

• To shed water properly, all horizontal joints in

sheathing paper must be lapped by at least 10 cm

(4”) with the upper layer installed to the exterior

side of the lower layer.

• Avoid unflashed horizontal butt joints at the top,

bottom, and/or corners of the window frame.

• Flashing above the window must direct water running

down the wall out and over the window. The top of

the flashing must be underneath the building paper.

• Windowsills must have drip edges that extend at

least 1 cm (1/2”) beyond the material below them.

• A sheet-metal pan or flexible waterproof membrane

under the window offers inexpensive insurance

against water seeping through window joints.

SummaryWindows are the weakest link in your home’s defence

against cold. Energy-efficient window technology is

proven and widely available for little or no extra cost.

Purchasers should ensure that all new or replacement

windows include energy-efficient features such as

low-e glass, argon gas fill, and insulated edge spacers.

Low-expansion foam used to air-seal gap between rough opening and window frame.

The ENERinfo Advisor Series

Brochures available in this series include:

• Building an Energy-Efficient Home

• Introduction to Home Heating

• Hot Water Answers

• Passive Solar Homes

• Ventilation for Older Homes

• Ventilation for New Homes

Other Information Sources

Canada Mortgage and Housing Corporation

www.cmhc-schl.gc.ca

Energy Star® Canada

http://oee.nrcan.gc.ca/energystar/

Office of Energy Efficiency

http://oee.nrcan.gc.ca/oee_e.cfm